platform/x86: acer-wmi: add another KEY_POWER keycode

Now that we have informed the firmware that the Power Button driver is active,
laptops such as the Acer Swift 3 will generate a WMI key event with code 0x87
when the power button key is pressed.

Add this keycode to the table so that it is converted to
an appropriate input event.

Signed-off-by: Antonio Rosario Intilisano <antonio.intilisano@gmail.com>
Acked-by: Gianfranco Costamagna <locutusofborg@debian.org>
Tested-by: Antonio Rosario Intilisano <antonio.intilisano@gmail.com>
Cc: Chris Chiu <chiu@endlessm.com>
Cc: Daniel Drake <drake@endlessm.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>